Rename target to DDRec, this required changes in many other CMakeLists to be consistent

Re-arrange Gear dependent parts into single if scope
Add SYSTEM to GEAR IncludeDirs

Use variables to define libraries in one place and add optional ones (i.e., gear) to the list

Create plugin_sources variable to create a DDRecPlugin independent of the DDRec library parts

No Whitespace after and before parenthesis
Assume ROOT_INCLUDE_DIRS is added in main CMakeLists file
Comment Setting of CMAKE_CXX_FLAGS

   shows all DetElements in model incl. #surfaces

   we need this application as it is the main simulation
   application for LC
 

   a DDG4 based simulation with xml configuration files
   (originally in DDSim)
 - renamed print_materials.cc to print_materials.cpp

 - added simple utility print_materials.cc 
 - started to implement averaging materials for surfaces
    - issue with ILDExDet geometry and assemblies ...

 - added drawing of LCIO events to teve_display 
   - so far only SimTrackerHits ( tbc) 

 - new method Surface::getVertices() (only planes so far)
 - added some convenient methods to Vector3D

DDCore and other packages now depend on DDSegmentation
Segmentation from DDCore now holds a DDSegmentation object that is created during XML parsing
Readout interface extended to act like a full IDDecoder combining information from the volume manage and the DDSegmentation object
TODO: Segmentation could be removed completely, if Readout simply holds a DDSegmentation object
TODO: Missing DDSegmentation classes for cylinders

    - DDExamples no longer built with DD4hep
      => will become standalone packages
    - moved UtilityApps from DDExamples to root level
      apps are built by default
    - create DD4hepConfig.cmake